Resin transfer molding is a closed mold, low pressure process applicable to the fabrication of complex, high performance composite articles of both large and small size. Several different resin transfer molding processes are well known to those skilled in the art. The process differs from various other molding processes in that a reinforcing material or preform such as glass fibers or other fiber reinforcement, is placed separately into a mold tool cavity. Resin is then injected under pressure into the mold cavity to combine with the preform to form a fiber reinforced plastic composite product.
Typically, a pre-shaped fiber reinforced preform is positioned within a molding tool cavity and the molding tool is closed. The mold is subsequently evacuated by pulling a vacuum through the mold. A feed line connects the closed molding tool cavity with a supply of liquid resin and the resin is pumped or "transferred" into the cavity where it impregnates and envelopes the fiber reinforced preform and subsequently cures. The cured or semi-cured product is then removed from the molding tool cavity.
The primary advantage of resin transfer molding resides in its capacity for high rate production. Although this process is widely known, the use of this molding process has not become widespread because of problems associated with the process. For example, use of the process has been hampered by the difficulties associated with stabilizing and de-bulking the dry composite preform and loading the same into the mold cavity. Maintaining adequate tolerances of the components and sealing the mold apparatus is also problematic.
Specifically with regard to molding aerospace structures, these difficulties often yield disoriented fibers, areas which are resin rich or lean depending upon bulk variations of a preform assembly, or porosity (i.e. trapped air due to an improper vacuum) within the composite which greatly diminishes the specific strength of the component. Due to the high costs of both fabrication and material, rejection of a single part out of a batch can negate the savings initially. As a result, use of the process has typically been limited to rather simple, low-strength components as compared to high strength aerospace components.
Thus, there is a continuing need in this field of art for means to obviate the above problems.